Smelting process of metallic silicon

Silicon metal smelting is a high-energy-consuming production. my country's silicon metal production has a long history. With the tightening of national energy policies, the development of energy conservation and emission reduction, and the promotion of new energy, metal silicon smelting has become a primary product and Technology, many domestic emerging energy companies have built a series of circular industrial chains such as metal silicon, polysilicon, monocrystalline silicon, solar cells, etc., which will inevitably affect the development of my country's entire energy field and the application of new energy in the next few years.

1. The necessity of producing silicon for chemical industry The metal silicon produced in my country (silicon content is mainly 98.5%) was originally mainly metallurgical silicon, and the production of chemical metal silicon (silicon content is mainly 99.85%) was mainly from the 1990s. Since the mid-term development, the output and export of chemical silicon in my country have grown rapidly. From 1999 to 2001, my country's chemical silicon exports to Japan reached 22,000 tons, 30,000 tons and 40,000 tons respectively. In 2001, my country's chemical silicon exports to Japan accounted for more than 40% of Japan's chemical silicon imports. . China has begun to join the ranks of chemical silicon producers and suppliers, and the number of companies producing chemical silicon is increasing. Since Shanghai Guangji Silicon Materials Co., Ltd. fully disclosed the smelting process of carbothermic reduction in 2002, the production capacity of silicon metal in China rapidly increased from 100,000 tons per year to 1.2 million tons between 2002 and 2004. As a result, the National Development and Reform Commission imposed comprehensive sanctions. In 2006, the actual output of silicon metal fell back to 700,000 tons. In 2006, only Dawu Silicon Factory of Shanghai Guangji Silicon Material Co., Ltd. successfully built a 10,000-ton-level metal silicon factory Jingxin Factory in the Xinjiang border. Others have built new silicon plants. Chemical silicon refers to metallic silicon used in the production of organosilicon and polysilicon. From a global perspective, the consumption of metallurgical silicon is more than that of chemical silicon. However, with the continuous development of science and technology, chemical silicon has been widely used in the production of organic silicon and semiconductors, and is widely used in the production of organic silicon. Silicon monomer and polymer silicone oil, silicone rubber, silicone resin building anticorrosion, water repellent, etc. They have unique properties such as high temperature resistance, electrical insulation, radiation resistance, and waterproofing. Used in electrical, aviation, machinery, chemical, pharmaceutical, national defense, construction and other departments. As the core of integrated circuits, more than 95% of electronic components are made of semiconductor silicon, which is the backbone of the contemporary information industry. Optical fibers in optical fiber cables, which are widely used in the "information superhighway", are also produced from metal silicon. The consumption of chemical silicon in the United States and the European Union has accounted for more than half of the total consumption of metal silicon. As a high-tech field and an important basic industry, chemical silicon is widely used, and its consumption tends to grow steadily. Under normal circumstances in the international market, the price of chemical silicon per ton is 300-400 US dollars higher than that of metallurgical silicon. Therefore, it is necessary to vigorously develop the production of chemical silicon, whether from meeting export and domestic demand, or from improving the economic benefits of metal silicon enterprises and improving product quality.

2. Raw materials for chemical silicon production In the production of chemical silicon, raw materials are a prerequisite for good operation. Quartz rock is used as raw material for producing chemical metal silicon, and low ash carbonaceous material is used as reducing agent. The raw materials for the production of chemical silicon by the electric furnace method mainly include silica and carbon raw materials. The carbon raw material is mainly petroleum coke, and there are high-quality anthracite or charcoal, and a part of it can also be mixed to increase the specific resistance of the charge. The raw materials are required to have the necessary purity and good reactivity in order to meet the product specifications; the reducing agent has different reactivity in order to fully react with the quartz stone; the charge has different components and has different particle sizes, In order to make the charging furnace have a good effect through proper coordination.

2.1. The process of smelting metallic silicon from silicon oxide minerals is a slag-free process. The selection of silica in chemical silicon smelting is stricter, not only the impurity content should be small, but also high mechanical strength, sufficient thermal stability and suitable particle size composition are required. Silica is preferably used for chemical smelting of silicon. Silica in its natural form exists either as an independent quartz mineral, or as a rock composed almost entirely of silica—silica, or as sandstone in the form of silica. During the smelting process, some impurities and adherents in the silicon oxide minerals contained in the production of chemical silicon are completely reduced, and some are partially reduced, and some enter the product silicon in the form of compounds or generate slag. This not only increases energy consumption and reduces product quality, but also causes difficulties in the smelting process. Therefore, strict requirements are imposed on the chemical composition of silicon oxide-containing minerals used in chemical silicon smelting. It is required that SiO2 is greater than 99%, Fe2O3 is less than 0.15%, Al2O3 is not greater than 0.2%, CaO is not greater than 0.1%, and the sum of impurities is not greater than 0.6%. The silica used must be washed with water before smelting, and the surface is clean. The silica entering the furnace is required to have a certain particle size. Silica particle size is an important process factor for smelting. The suitable particle size of silica is affected by various factors such as the type of silica, electric furnace, capacity, operating conditions, and the type and particle size of the reducing agent, which should be determined according to the specific smelting conditions. In general, a 6300KVA three-phase electric furnace (built in Dawu Silicon Factory in 1983) requires a silica particle size of 8-100mm, and a 3200KVA three-phase electric furnace requires a silica particle size of 8-80mm, and the proportion of the intermediate particle size is larger. When the particle size is too large, it is easy to make unreacted silica enter into the liquid silicon due to the incompatibility with the pounding material and the reaction speed, resulting in an increase in the amount of slag, difficulty in releasing the furnace, a reduction in the recovery rate of silicon, an increase in energy consumption, and even an increase in furnace The bottom rose, affecting normal production. If the particle size is too small, although the contact surface of the reducing agent can be increased, which is beneficial to the reduction reaction, the gas produced during the reaction cannot be discharged smoothly, and the reaction speed will be slowed down. The particle size is too small. The impurities brought in will increase again, which will affect the product quality. Silica that is generally less than 5mm in production should not be used.

2.2 Carbonaceous reducing agents The main reducing agents used in chemical silicon smelting are petroleum coke, bituminous coal and charcoal. In order to increase the resistivity of the charge and increase the chemical activity, it is also matched with gas coal coke, silica coke, blue carbon, semi coke, low temperature coke and wood block. In the chemical composition of carbonaceous reductants, fixed carbon, ash, volatile matter and moisture should mainly be considered. Generally, the fixed carbon is required to be high, and the total amount of reducing agent required is reduced, so that less impurities are brought into the ash, the amount of slag is correspondingly reduced, the power consumption is reduced, and the impurity content in the chemical silicon is reduced. The resistivity of the carbonaceous reducing agent should be large and the porosity should be high. The charge resistivity mainly depends on the carbonaceous reducing agent. The carbonaceous reducing agent has high resistivity, good chemical activity and high recovery rate of silicon. Petroleum coke has the lowest ash content among the reducing agents used in the production of metal silicon, with ash content of 0.17-0.6%, fixed carbon of 90-95%, and volatile content of not more than 3.5%-13%. Chemical silicon smelting uses petroleum coke as a reducing agent, because its low ash content is conducive to improving product quality. However, due to the small resistivity and poor reactivity of petroleum coke, it is easy to graphitize at high temperature. When the dosage is too large, the furnace condition is not well controlled, resulting in non-sintering of the charge, serious stab fire, high power consumption, and difficulty in firing. Charcoal has high specific resistance and reactivity, and has low impurity content. It is an ideal reducing agent for smelting industrial chemical silicon. However, the properties of charcoal prepared by different woods and different methods are also very different. The ash content of peeled charcoal is usually one-half to one-third lower than that of barked charcoal, and the bark has a large effect on the ash content of the charcoal. The main component of charcoal is carbon, and the ash content is low, generally less than 10%. High resistivity and good chemical activity. Years of production practice have proved that charcoal is an important carbonaceous raw material to meet the needs of smelting chemical silicon, but the source of charcoal is limited, and charcoal reducing agent can no longer be used. Judging from the situation abroad, most countries no longer use charcoal. Many domestic manufacturers have also done a lot of work in seeking and using charcoal substitutes. Practice has proved that among various carbonaceous reductants, bituminous coal is another ideal reductant besides charcoal in terms of reactivity and specific resistance. Bituminous coal is characterized by high resistance and strong reaction ability, and low ash bituminous coal is obtained by washing. The ash content can reach about 3%, the Fe2O3 content is 0.2-0.3%, and the Al2O3 content is less than 1%. The ash content of the reducing agent bituminous coal in my country is more than 3%, while the ash content of the foreign reducing agent bituminous coal is more than 1%. The function of the wood block is to increase the resistance of the material layer, and the size of the amount has an impact on the furnace condition. If the amount of wood block is too large, the material layer will be loose, the furnace condition will deteriorate, and the power consumption will increase. Due to the low ignition point and low carbon content of the wood block, it is actually very little as a reducing agent. The impurities in the carbon raw material are mainly ash, which are all composed of oxides. In chemical production, the oxides in the ash are also reduced, which consumes both electricity and carbon, and the reduced impurities are still mixed into the silicon liquid, reducing the strength of silicon. In production practice, every 1% increase in ash content in the charge will consume 100-120 degrees more electricity. Therefore, the less ash content in the carbon raw material is required, the better.

2.3 Electrodes Electrodes are one of the main consumable materials in the production of chemical silicon. Electrodes for chemical silicon smelting generally use graphite electrodes and carbon electrodes, and graphite electrodes are mainly used in China. In a silicon smelting furnace, the electrode is the heart and an important part of the conductive system. The current is input into the furnace through the electrode to generate an arc for chemical silicon smelting. Requirements for electrode materials: (1) Good conductivity and low resistivity to reduce power loss. (2) The melting point should be high, the thermal expansion coefficient should be small, and it is not easy to deform; (3) It has sufficient mechanical strength at high temperature and low impurity content. Graphite electrodes have low ash content, good electrical conductivity, heat resistance and corrosion resistance, and are the best choice for chemical silicon smelting.

3. The smelting process of silicon for chemical industry The technological process of chemical silicon includes charge preparation, electric furnace smelting, silicon refining and casting, and crushing by removing slag inclusions. All raw materials are subject to necessary treatment prior to charge preparation. Silica is crushed in a jaw crusher to a degree of not more than 100mm, and the fragments smaller than 5mm are screened out and washed with water. Because the fragments in the furnace are melted in the upper part of the furnace, the gas permeability of the charge is reduced, making the production process difficult. Petroleum coke has a high electrical conductivity, and it is necessary to crush it to a degree of not more than 10mm, and to control the amount of petroleum coke powder. Because it burns directly on the furnace mouth, it will cause insufficient reducing agent. In the production of chemical silicon, bituminous coal can completely replace charcoal. For example, Hunan Zhuzhou finely washed bituminous coal has a fixed carbon of 77.19%, a volatile content of 19.4%, an ash content of 3.41%, a Fe2O3 content of 0.22%, an Al2O3 content of 0.99%, and a CaO content of 0.17%. . Through production practice, it is feasible to use this bituminous coal to smelt chemical silicon. Wood blocks and wood chips for the production of chemical silicon are processed with wood cutters and wood chippers. The carbonaceous reducing agents in the charge are mainly petroleum coke and bituminous coal, and the amount of wood blocks and wood chips depends on the furnace conditions. No wood is used in production, but the product quality is more stable. The proportion of the charge is determined according to the product level required to be produced. The ratio of petroleum coke and bituminous coal is determined according to the amount of carbon required for each batch of mineral silicon. The ratio of petroleum coke and bituminous coal has a great influence on the working resistance of the charge. After each component of the charge is weighed, the charge is mixed evenly, and after pounding, the evenly mixed charge is concentrated into the furnace. Keep a certain material surface height and feed evenly. Chemical silicon production is continuous. The conditions inside the furnace are not permanent either. Chemical silicon production is a process in which electrical energy is converted into heat energy in an electric furnace, and then the heat energy is used to directly heat the material to produce a chemical reaction. Therefore, the electrical characteristics of the furnace are very important. The smelting implements closed-arc operation, maintains the high temperature furnace, improves thermal efficiency, and improves the utilization rate of the electric furnace. Smelting is carried out by the operation method of simmering for a certain period of time and regular centralized feeding. Under normal circumstances, it is difficult for the charge to sink automatically, and it is generally necessary to force the charge to sink. Furnace conditions fluctuate easily and are difficult to control. Therefore, it must be judged correctly in production and dealt with in time. Every 4 hours, the furnace is released, refined and cast, crushed and slag sorted and stored.

4. Electric furnace operation Chemical silicon smelting is carried out in submerged arc state. In order to produce a constant homogeneity of metallic silicon, optimum furnace operation is required during the smelting process. The main source of heat is electricity. Therefore, the current flow path in the furnace and the current distribution of each route have an important impact on the temperature distribution of each zone in the furnace and the progress of the entire smelting process. Pay attention to maintaining the load balance of the three-phase electric energy, so as to increase the output. Ensure quality and reduce power consumption.

4.1 Feeding and pounding In order to make the silicon smelting furnace achieve the purpose of high quality and high yield, in addition to requiring good electric furnace parameters, excellent raw materials, and reasonable ratio, the quality of the operation method is a very important factor. A reasonable feeding method plays a leading role in the structure of the material layer, the stability of the electrode in the furnace and the full utilization of thermal energy. In production, feeding and stoking are combined. According to the different conditions and characteristics of the smelting process, the feeding and stoking operations should be completed in a timely manner. In order to maintain good air permeability in the furnace, it is necessary to dazzle and pound the furnace. The small pounding furnace is carried out according to the furnace conditions, and the large pounding furnace is generally carried out once every hour or so. It is not advisable to carry out the smelting operation in one electrode area and one electrode area. It is necessary to concentrate and unify the stoking and charging, so as to maintain a higher smelting temperature. The most important factor in stabilizing furnace operation is maintaining a constant temperature distribution in the bed. Furnace operation can be severely disrupted if the furnace chamber temperature distribution is disrupted [. The particle size and uniformity of the charge in production, the charging and discharging, and the treatment of the furnace material surface will affect the movement of the electrode. Excessive electrode movement and strong stoking can cause unstable furnace operation.

4.2 Closed arc operation The closed arc operation is to properly bury the electrode in the charge, and use the semi-molten charge as an impedance to generate an arc between the electrode and the molten charge. In order to achieve closed arc operation, we must first consider the feeding method. Feeding methods include one-time feeding method, batch feeding method and multiple feeding method. Except for the one-shot feeding method, which is an open-arc operation, other methods can achieve closed-arc operation. In the production of chemical silicon, we adopt the batch feeding method, which has stable material layer structure, low power consumption and long furnace life. There are several problems to be dealt with in the operation: one is to choose suitable electrical parameters so that the electrode can be inserted into the material layer at an appropriate depth; the other is to find a way to control the specific resistance of the charge; the third is that the particle size of the charge has an impact on the smelting of silicon. Important influence, too large and too small particle size are detrimental to furnace conditions. The advantages of closed arc operation are: ①The structure of the material layer in the furnace can form a complete system, and the charge sinks in turn; ②The arc light is not exposed, the radiant heat loss on the material surface is greatly reduced, the blast furnace temperature is maintained, and the thermal efficiency is improved, thereby increasing the output, Improve product quality and reduce power consumption. ③It can make the electrode consumption reach a balance and stability, and avoid the accident of worrying about breaking. ④The temperature of the material surface is relatively low, so that the equipment on the material surface is less subject to thermal corrosion, which prolongs the life of the equipment and improves the utilization rate of the electric furnace equipment. ⑤ Less dust can make the furnace surface operation have a better operating environment. Regardless of the size of the furnace, as long as appropriate measures are taken, arc closed operation can be achieved to obtain ideal production results.

4.3 Power distribution technology The electric arc furnace is a device that uses the heat generated by the arc to heat. In the process of chemical silicon smelting, the physical and chemical changes are closely related to the electrical system. The power distribution operation is good or bad, yes Smelting efficiency has a very important impact. The arc mainly exists at the electric end, and the cavity is affected by the impact thrust of the arc, and the material legs are opened to form a bulb shape. During the smelting process, the control of the electrical parameters of the electric furnace is completed by the power distribution work. In general, it is the buried depth of the control electrode. Shallowly buried electrodes generally indicate that the reducing agent is excessive, a fire eye is formed near the electrode, the arc is loud, the silicon temperature is low, the quantity is small, and the power consumption is high. The electrode is deeply buried, if the reducing agent in the charge is too low, the electrode will be in a lower position. Because the charge resistance increases with the reduction of carbon in the charge, the increase in resistance reduces the current load, the electrode consumption increases, and the productivity decreases. In production, the buried depth of electrodes is determined according to field operations. Adjusting the electrode buried depth is to change the resistance value of the charge, which is the best way to adjust the furnace condition. When the secondary voltage of the electric furnace exceeds a certain value, the electrodes will be damaged, the volatilization loss of silicon will increase, the upper part of the furnace will be overheated, and the heat loss will increase. The secondary current is limited by the allowable current density of the electrode and cannot be increased arbitrarily. The ratio of current to voltage is an important factor in the operation of the furnace. If the current-voltage ratio is too small, the electrode cannot go down, and the open arc production is difficult to operate. The current-voltage ratio is too large, the electrode is inserted too deep into the charge, and the production is not ideal. In production, only when the proper current-voltage ratio is found, the working current is stable, the material is balanced and the electrode is raised and lowered, the best production results can be achieved. Adjusting the working voltage is an important means to adjust the productivity of the furnace. The working voltage of the furnace depends on two aspects: one is the short network structure, which requires high electrical efficiency and appropriate power factor. On the other hand is the furnace condition, including the furnace structure and production operation. The resistance value of the working resistance in the smelting process is very important, it is easy to change, and efforts should be made to make it stable and approach the optimal value. In general, in order to ensure normal material surface temperature, increase the voltage. The normal surface temperature of the bag is around 600°C. Using raw materials that meet the specifications, the particle size of the charge is large, the resistance is small, the branch current is large, and the electrode is not easy to penetrate.